In recent years, much attention has been paid to developing energy absorbing front and rear vehicle bumper structures to absorb low speed direct frontal collisions, i.e. in the order of one to five mile per hour. These bumper systems are generally supplemental to other passenger restraint systems such as seatbelts and steering column mounted expandable air bags, with the air bag systems being primarily active for the higher speed impacts exceeding the one to five mile per hour range.
The typical low impact energy absorbing bumper system in popular use is the "soft" bumper system. This generally includes a flexible synthetic resin front bumper plate enclosing an energy absorbing type foam material, all of which may be mounted on a fairly rigid back plate or cross member extending transversely of the vehicle and mounted at its ends to axially extending vehicle frame members. Such a cross member is usually made of steel and is in effect the rigid chromed steel bumper which was so popular in the past. Typical of these soft bumper systems are those shown in U.S. Pat. Nos. 3,574,379, 4,542,925, 4,569,865 and 4,762,352.
Additionally, it is known to provide a fairly rigid bumper system with axially directed shock absorber members located intermediate the bumper structure and the vehicle frame to allow the entire bumper structure to yield upon receiving a relatively low energy impact and the return to its initial position upon cessation of the impact load.
Finally, internal stroking type bumper beam constructions are known wherein a fairly rigid front bumper plate member is retained on a face plate or similar member located at opposite ends of the bumper beam in such a manner that the front bumper beam can slide rearward toward the face plate upon receiving a frontal impact. An energy absorbing material is located between the front bumper beam and face plate to absorb the energy of the impact and return the front bumper beam to its original position upon cessation of the impact. Such a system has been proposed as an add-on to the vehicle frame members, as shown in U.S. Pat. No. 4,460,205.
Despite the foregoing developments, there exists the need for an energy absorbing vehicle bumper system which is designed as an integral part of the vehicle frame structure, is simple in construction with a minimal number of component parts, provides in the unloaded condition a bumper structure having minimal length in the fore-aft axis of the vehicle, and is of minimal weight yet capable of resiliently absorbing a maximum amount of impact energy per unit stroke.